A common process implemented during fabrication of semiconductor devices is photolithography or lithography. For this process, light may be radiated through a patterned mask onto a substrate covered with a light-sensitive or photosensitive material, e.g., photoresist. The radiated light can be in the ultra-violet (UV) wavelength range for exposing the photosensitive material. Radiating light to expose portions of the photosensitive material changes the chemical composition of those portions. Such a chemical composition change can allow the exposed portions to be removed faster than the unexposed portions, e.g., when using positive photoresist. Alternatively, the chemical composition change can make the exposed potions more difficult to dissolve, e.g., when using negative photoresist. In either case, the photosensitive material that remains after development is typically used as a protective mask in subsequent fabrication steps.
As semiconductor devices become more advanced and sophisticated, the patterned structures etched on a substrate are becoming smaller and more dense. One problem arises from the difference in optical behavior at dense patterns and small iso spaces. This phenomena may also be known as the “proximity effect” or “optical proximity effect.” This effect occurs when radiated light through a patterned mask for exposing photosensitive material is scattered, which can cause undesirable exposure results. For example, in creating dense patterns of a particular target dimension and optimal exposure energy, this effect can cause incomplete exposure or under-exposure of the photosensitive material in the small iso spaces between unexposed potions. As a result, the incomplete or under exposed portions of photosensitive material in the small spaces after development may turn into “scum.”
Scum may be avoided by redesigning the mask used to create the pattern by using optical proximity correction (“OPC”) operations well-known in the prior art. However, OPC is an expensive and time consuming undertaking. Additionally, it requires the creation new masks, which can be expensive. Scum may also be avoided by adjusting the numerical aperture (NA) and/or coherence of the illuminator (Sigma) for the light radiation source in an effort to expose the small iso spaces between the intended unexposed portions of the photosensitive material. However, adjustment of NA and Sigma may result in the creation of lithography patterns with dimensions smaller than designed and because of differing distortion, which in turn can lead to conducting layer dimensions that are smaller than intended. Such smaller dimensions can lead to higher resistances, thereby slowing signal speeds. Alternatively, scum may be removed by manual repair methods known in the art, such as laser repair. However, the afflicted portions may be too large and the defects too pervasive for such repairs to be cost-effective and efficient.
Another problem with forming dense or narrow line patterns arises from the fact that their dimensions are smaller than the default minimum design rule for next generation product test patterns. Such dense and narrow line patters are more prone to peeling of the photosensitive material due to the small surface area available for the material to adhere to the substrate. For example, such situations may arise in “drop-in”areas- non-essential areas of a semiconductor device sometimes used for experimenting and testing semiconductor structures. These test structures may-have smaller lines and space patterns than the structures found in the essential areas, while the thickness of the photosensitive layer remains the same. This situation makes the photosensitive material in the dense and narrow line areas especially susceptible to peeling. Peeling of photosensitive material creates debris that may cause serious damage to a semiconductor device in subsequent fabrication steps.
Thus, what is needed are improved photolithography or lithography processes that avoid forming scum and peeling of photosensitive material.